Replacement functions, such as names<-, seem to not use lazy evaluation when called like names(x) <- c("a", "b").

To demonstrate, let’s define a function to get the fractional part of a number and a corresponding replacement function – but inside the replacement function, include a line to print the defused value argument.

fractional <- function(x) { 
  x %% 1 
}

`fractional<-` <- function(x, value) {
  print(rlang::enexpr(value))
  invisible(x %/% 1 + value)
}

Now if we call fractional<- directly, it prints the expression we gave for value:

x <- 10.1
`fractional<-`(x, 0.2 + 0.2)
#> 0.2 + 0.2

But if we call it like in the assignment form, it prints the result of evaluating the expression:

x <- 10.1
fractional(x) <- 0.2 + 0.2
#> [1] 0.4

The language definition explains replacement functions like:

names(x) <- c("a","b")

is equivalent to

`*tmp*` <- x
x <- "names<-"(`*tmp*`, value=c("a","b"))
rm(`*tmp*`)

But this doesn’t reproduce this behavior:

x <- 10.1

`*tmp*` <- x
x <- "fractional<-"(`*tmp*`, value=0.2 + 0.2)
rm(`*tmp*`)

#> [1] 0.4

What is happening internally in <- that makes it so that value is passed to fractional<- after being evaluated, and is there any way to circumvent this behavior?